The above mentioned type of outrigger jack has been used to securely support the vehicle body of a maintenance vehicle such as an aerial work platform machine or a crane truck, while the vehicle is used for performing some work. Japanese Utility-Model Publication No. H6(1994)-24290 discloses an example of such prior-art outrigger jack, which is shown in FIG. 8. This outrigger jack 50 comprises an outer post 51, which is fixed to the frame of a vehicle (or to an outrigger beam), an inner post 52, which is telescopically inserted into the outer post 51 and extensible downwardly, an extension cylinder 55, which is provided inside these posts 51 and 52, and a ground plate 60, which is mounted pivotally on the lower face plate 53 of the inner post 52. The extension cylinder 55 comprises a cylinder tube 56 and a cylinder rod 57, which is fitted in the cylinder tube, and the cylinder rod 57 is actuated by the hydraulic pressure supplied into the cylinder tube 56 for the extension and contraction of the extension cylinder.
The cylinder tube 56 is connected pivotally to the outer post 51 while the lower end 57a of the cylinder rod 57 is connected pivotally to the lower face plate 53 of the inner post 52 with a pivot pin 54. Thus, the inner post 52 is extensible downward with respect to the outer post 51 by the actuation of the extension cylinder 55. On the lower side of the lower face plate 53 of the inner post 52, a U-shaped member 58 is welded. This U-shaped member is engaged with an inverted U-shaped member 59, which is welded to the upper face of the ground plate 60 as shown in the drawing. In this construction, the ground plate 60 is connected pivotally to the inner post 52, so it swings freely. In other words, the U-shaped member 58 and the inverted U-shaped member 59 together function as a universal coupling.
In this construction of the outrigger jack, when the inner post 52 is moved downward, the ground plate 60 is brought into contact with the ground to support the vehicle body. In this condition, the force of the cylinder rod 57 which pushes downward acts on the pivot pin 54, the lower face plate 53 of the inner post 52, and the U-shaped member 58 and then on the ground plate 60. As understandable from this, the force of the extension cylinder 55 is transmitted to the ground plate 60 through a number of members. Thus, this outrigger jack, which has such a complicated construction, is inefficient for transmitting the force necessary for the secure support of the vehicle body.